Sounds like the oil pump sucked up something it couldn't digest and siezed up. Did you take the time to disassemble the oil pump, clean it and blueprint it before installing it? Oil pumps are notorious for coming off the production line full of metal dust and shavings, so even if you don't blueprint it, you still should pull apart a new pump and clean it very well with something like brake cleaner then mineral spirits to get all the brake cleaner out.
Also, it's not a bad idea to 'prime' the oil pump by giving all the parts a nice coating of assembly lube while you have the pump apart.